1. Technical Field
The invention relates to tapping metal through an electrolyte layer which is lighter than the metal, and particularly, where the metal is aluminum.
2. Description of the Prior Art
Aluminum is typically produced in electrolytic cells operated at currents of up to 300,000 amps or more, between carbon anodes and a carbon cathode. The carbon cathode forms the floor of a container with sidewalls of carbon or refractory, surrounded by insulation and contained within a steel shell. Within the container is a lower layer or pool of molten aluminum on the carbon cathode floor and an upper less dense layer of molten electrolyte (sodium/aluminum/fluoride salt) lying on top of the aluminum, thus the layers form a liquid-liquid interface between the upper and lower layers. The sidewalls generally are covered with a layer of frozen electrolyte which can extend down and cover the outer periphery of the cathode surface. The exposed top surface of the electrolyte is generally covered by a crust which comprises a mixture of electrolyte and aluminum. The carbon anodes are immersed in the electrolyte and are positioned with their bottom faces a few centimeters (typically less than 5 cm) from the electrolyte metal interface. The molten aluminum layer is typically between 12 and 20 cm. thick, and the electrolyte layer is typically about 20 cm. thick. During operation, alumina is dissolved in the electrolyte and is electrolyzed by direct current flowing from the anodes to the cathode to form more aluminum at the molten metal surface.
The density of the electrolyte is only slightly less than that of the molten aluminum and the interface between the electrolyte and the molten aluminum is relatively unstable and can easily be disturbed.
The metal produced in the electrolytic cell is periodically tapped or withdrawn from the metal pool by inserting a hollow metal pipe, usually fabricated in cast iron, through the electrolyte layer into the metal pool. This pipe or tube is operatively and pneumatically connected to a collecting or tapping crucible. A vacuum is applied in the gas phase of the crucible and this vacuum pulls the metal produced in the cell into the crucible through the pipe where the metal is collected. The metal pipe is often referred to as the “tapping siphon”. The operative end immersed in the electrolyte and metal is often called the “siphon tip”. It should be noted that although the term siphon is used, the action of withdrawing the metal from the electrolytic cell is due to the application of a vacuum in the gas phase of the crucible and is not due to the action of a siphon. When metal is tapped from a cell, an amount based on a predefined target is removed. The target is based on the estimated metal production rate between tapping operations. Typically the tapping crucible is designed with a capacity sufficient to permit tapping several cells (such as three or four cells) and thus the metal from these cells is mixed in the tapping crucible. When the tapping crucible is full, it can be emptied into a holding furnace which can contain the contents of a number of tapping crucibles. In some operations, metal may be transferred first to an intermediate crucible before transferring to the holding furnace.
Due to the rather shallow depth of the metal pool in the electrolytic cell, a problem arises if the molten metal is not withdrawn carefully. If sufficient care is not taken, electrolyte from the electrolyte/metal interface may be withdrawn along with the metal into the tapping crucible. This electrolyte causes deposits in the crucible and contamination in the holding furnace fed from the tapping crucible. “Visualization of Tapping Flows”, by M. L. Walker, Light Metals, The Minerals, Metals and Material Society, edited by Reidar Huglen, pages 115 to 219, 1997, describes a study of the effect of the suction rate on the electrolyte/metal interface.
Walker describes tests done in a “water model”, where the electrolyte and the metal in an electrolytic cell are simulated by immiscible liquids having appropriate densities. In this particular study, the two layers were quiescent (not circulating or flowing). By inserting a hollow pipe below the interface between the liquids and withdrawing liquid, Walker concludes that increasing the flow velocity in the hollow pipe causes the interface to be drawn downwards where it eventually was drawn into the pipe interior. From this study, Walker concluded that increasing the flow velocity in the pipe caused “entrainment” of the material above the interface, and therefore in a real electrolytic cell would cause electrolyte to be drawn into the pipe used to tap the electrolytic cell thereby contaminating the metal being tapped. The contact of electrolyte being thus drawn into the pipe with the metal and adjacent cathode floor tends to erode the cathode floor. Walker proposes increasing the interior cross-section of the bore of the pipe placed within the metal, generally expanding the normal circular cross-section bore to an elongated elliptical shape. This is intended to reduce the metal flow velocity as it enters the bore in the pipe to reduce the tendency to draw electrolyte into the pipe. However, this requires an enlarged opening in the tapping pipe which is more difficult to use industrially. Furthermore, the solution is based on a “quiescent” metal and electrolyte layer, which is not representative of real cell operations.
It has been found that a further problem during withdrawal of metal is that the amount of entrained bath varies widely from cell to cell and even on subsequent removals from the cell. This may be caused by many factors including variability of metal depths, location of freeze, and presence of sludge. In some cases, more entrained bath may be present at low removal rate than at high removal rates. Therefore, simply reducing the rate of removal is not an effective solution to the problem.